Support for download to save multiple files

If `download_all` is set, all urls in `urls` will be downloaded rather than stopping at the first successful one. Filenames can be stored in `filenames`, temp files are `files` and output files are `outputs`. 
Uses `filename` as a directory to save the files into within the download path, and sets `output` to this folder on completion.
This is mostly for discussion and ideas. I decided to go for a mode for download rather than a separate plugin as this allows other plugins to control whether one or multiple items are downloaded, rather than forcing the user to know which download plugin to use when.